Cardiac pacemakers, cardioverters and defibrillators are well known in the art and provide important life-saving treatment and safeguards for many patients. Such implantable medical devices have long been utilized to treat patients prone to suffering ventricular or atrial arrhythmias such as ventricular tachycardia and ventricular fibrillation. Once implanted in the patient's body, the cardiac pacemaker, cardioverter or defibrillator monitors the patient's heart. If the heart enters fast ventricular tachycardia or ventricular fibrillation, the cardioverter/defibrillator may deliver cardioversion therapy to shock the heart out of the tachycardia or fibrillation and return the heart to normal sinus rhythm.
Determining which patients may be effectively served by the implantation of an implantable cardioverter/defibrillator may be difficult. Historically, only patients who had previously suffered ventricular fibrillation were implanted with a cardioverter/defibrillator. Subsequent clinical testing and clinical trials have provided expanded indications for patients who may benefit from a cardioverter/defibrillator. However, these indications have typically been limited to patients who had suffered a previous medical condition, such as a myocardial infarction or heart failure. As such, a substantial portion of the population which has never suffered a ventricular fibrillation episode or other traumatic cardiac event has relatively few means for being indicated for an implantable cardioverter/defibrillator.
It is known, though, that patients who have never suffered a prior cardiac episode may still experience a ventricular or atrial arrhythmia such as ventricular tachycardia or ventricular fibrillation. Research has been directed toward analyzing cardiac signals to identify characteristics indicative of an increased propensity toward suffering ventricular or atrial arrhythmia such as ventricular or atrial tachycardia, or ventricular or atrial fibrillation and sudden cardiac death. Such characteristics include, for instance, the electrophysiological properties of cardiac tissue or triggers that may tend to lead to ventricular tachycardia or ventricular fibrillation. However, the results of such research has proven only partially successful, as the results of the studies have tended to show that a particular cardiac characteristic will tend to show only one aspect of the underlying cause of a future ventricular or atrial arrhythmia such as ventricular tachyarrhythmia or ventricular fibrillation. Thus, the tests based on cardiac characteristics have tended to provide a substantially incomplete estimation of the patient's likelihood of suffering a ventricular or atrial arrhythmia such as ventricular tachycardia or ventricular fibrillation.